1. Field of the Invention
This invention relates to a solid state imaging device, and more particularly to a solid state imaging device based on free carrier absorption formed by silicon and having a capability of detecting incident light over a wide wavelength range.
2. Description of the Prior Art
For example, there is a conventionally used a solid state camera device (including Charge Coupled Device: CCD) of a four-phase transfer type having a composition as shown in FIGS. 1 and 2A. FIG. 1 shows a plan diagram of the conventional CCD, and FIG. 2A shows a cross sectional view along the line X--X in the CCD as shown in FIG. 1. FIGS. 2B and 2D are potential energy diagrams of the CCD as shown in FIG. 2A.
In the FIGS. 1 and 2A, a plurality of first insulating region 2 having a predetermined distance between each other are formed on a surface portion of a P-type semiconductor substrate, such as a silicon substrate 1, and a plurality of photo diodes 3 formed with the insulating region 2 and the silicon substrate 1 are arranged as shown. On the substrate 1, a plurality of second insulating regions 4 as a charge transmit channel line 4 (channel region) of the CCD having a predetermined interval to each other are formed.
On the substrate 1 between the first insulating region 2 and the second insulating region 4, a transfer electrode 5 which transfers signal charges accumulated in the photodiode 3 to the channel region 4 (the second insulating reagion 4) is formed continually along a predetermined direction. Moreover, a plurality of transmit electrodes 6 are formed for transferring the signal charges outside of the solid state imaging device. Four transmit electrodes 6 are formed in a ratio of per a photodiode 3.
In the solid state camera device having the above mentioned composition, incident light is transformed photoelectrically in the photodiode 3. The signal charges transformed photoelectrically are proportional to the amount of incident light accumulated in the photodiode 3, as shown in FIG. 2B. Under the state in which a conduction band at P-N junction is poor in free electrons, the photodiode 3 catches the signal charges caused by a fundamental absorption, which means that electrons risen by absorption of the incident light are transferred from valence band to conduction band and then accumulated. When an electric-potential barrier formed between the photodiode 3 and the channel reagion 4 is increased to a high level by supplying a predetermined voltage to the transfer electrode 5, the signal charges accumulated in the photodiode 3 are transferred to the channel region 4, as shown in FIG. 2C. After the signal charges are transferred to the channel region 4, the electric-potential barrier of the transfer electrode 5 is decreased to a low level by stopping to supply voltage to the photodiode 3, as shown in FIG. 2D, so that the electric potential barrier is formed again in the region between the photodiode 3 and the channel region 4. While, the signal charges transferred in the channel region 4 are transferred to predetermined direction by continually supplying transfer signals to the transmit electrode 6, and then outputted outside of the CCD by a voltage-transform in an output circuit (not shown).
By operating the above mentioned way repeatedly, the incident light is transformed to signal charges by a light-electric transform method and then read out at the outside of the CCD.
As has been described, in the conventional CCD in which the incident light is transformed to signal charges by the light-electric transform method, the signal charges are generated by the transferring electrons from a conduction-band to a valence-band, namely triggering by the fundamental absorption of light in semiconductor. When the energy of incident light is only larger than that of a forbidden band in a semiconductor by which a photodiode is formed (generating signal charges and accumulating them), fundamental absorption occurs and signal charges are generated. Accordingly, in a case of such the photodiodes are formed by the P-N junction in the silicon such as the conventional CCD as shown in FIGS. 1 and 2A, incident light having an energy larger than a energy of a forbidden band width in silicon is only detected by the photodiode. Namely, there is a problem in that it is difficult to detect the incident light having a long wavelength more than 1 or 2 .mu.m by the conventional CCD.